The present invention relates to a method of fabricating field effect transistors, and more particularly to a method of fablicating a gate of a field effect transistor made of semiconductor compounds such as GaAs.
Metal-Schottky gate field effect transistors (MESFET) have been well known in the art as having excellent properties of high frequency. The Shottky gate field effect transistors have gates formed by the Schottky contacts or metal-semiconductor contacts. They have many applications due to their high frequency performance to micro-wave low noise amplifier devices, high power amplifier devices, and oscillators. The properties and reliabilities of the Schottky gate field effect transistors strongly depend upon the quality of the Schottky gates. The quality of the Schottky gate further depends upon the height of the potential barrier appearing at the metal-semiconductor contact or the Schottky contact in which carriers experience such potential barrier when moving across a face defined by the Schottky contact from the metal region into the semiconductor region. Such potential barrier will hereinafter be referred to as a potential barrier .phi..sub.B. The potential barrier .phi..sub.B is ideally associated with the number given by subtracting the affinity of electrons from the work function .phi..sub.m of metal, namely ".phi..sub.m -.chi.". Actually, the potential barrier .phi..sub.B is defined by a Fermi level pinning and thus depends primarily upon an interface state rather than the work function of the metal. It was known that in the case of GaAs as a semiconductor compound, the potential barrier .phi..sub.B is relatively large, for example, approximately 0.7 V to 0.9 V which is suitable to form the Schottky contact.
In addition, the GaAs Schottky gate field effect transistor has the following advantages. That transistor exhibits a relatively small surface recombination and thus has a desirable ideality factor "n" which is near to 1. The Schottky contact in the GaAs transistor both has an interfacial structure exhibiting a thermal stability and has a relatively small resistance. Further, the GaAs compound semiconductor has a strong adhesion with a substrate, a low stress, a high heat resistivity and a facility in micro-lithography.
So far as the material of the gate electrode of the MESFET is concerned, Aluminum is useful as having a small resistivity but having high heat resistivity and reliability, in addition to its ease of manufacture. The well known technique such as a lift-off using an electron beam evaporation is available to make the Al gate.
The Al gate is, however, engaged with a disadvantage in appearance of the electro-migration causing voids in the Al metal. This renders the reliability of the Al gate to become increasingly low as the minimization of the device size represented by a submicron gate length is realized. Further, it appears that a native oxide film is formed on a surface of the Al gate thereby vendering electrical connection inferior. But at present, there seems to be no material which can replace aluminum as the Schottky gate. In any event, in the art the GaAs MESFET having the Al Schottky gate are influential.
A conventional method of fabricating the GaAs MESFET having the Al Schottky gate will be described with reference to FIGS. 1A to 1D.
With reference to FIG. 1A, a semi-insulating GaAs substrate 1 is prepared to form a GaAs MESFET. A channel layer 2 or an active layer is formed on the semi-insulating GaAs layer by epitaxial growth or ion-implantation. An insulating film 3 such as silicon oxide film SiO.sub.2 is deposited on the channel layer 2. The insulating film 3 is subjected to a selective etching thereby an opening is formed in a gate formation region on which a gate will be formed. The opening of the insulating film 3 is defined by a gate length L.sub.g.
With reference to FIG. 1B, a metal film 12 having a heat resistivity is deposited on the entire surface of the device, and thus both on the surface of the insulating film 3 and in the opening thereof. The metal film 12 may be made of tungsten W, molybdenum Mo and tungsten silicide WSi.sub.X. A part of the metal film 12 will become a Schottky gate electrode of the MESFET. To reduce the gate resistance, a secondary metal film 6 made of a metal having a low resistivity such as gold Au is further deposited on the entire surface of the metal film 12. So far as the combination of the two metal layers 12 and 6 is concerned, it is possible to insert a titanium Ti film or a platinum Pt film between the metal layers 12 and 6 in order to prevent any diffusion between the tungsten silicide WSi.sub.X and the gold Au and further to improve the adhesion thereof. For example, it is useful to form a metal multi-layer comprising Ti-Pt-Au on the heat resistive layer 12.
With reference to FIG. 1C, a photo-resist film 10 is patterned to be used as a mask pattern for etching the metal film 6 of Ti-Pt-Au by use of an ion-milling method. Further, according to the mask pattern 10, the heat resistive metal film 12 of the tungsten silicide WSi.sub.X is selectively etched by a dry etching using a CF.sub.4 gas. As a result, the gate electrode comprising the heat resistive WSi.sub.X film 12 and the low resistive metal film 6 is defined. The photo-resist film 10 is stripped thereafter.
With reference to FIG. 1D, contact holes are formed in the insulating film 3. An Au-Ge-Ni alloy is deposited by a vacuum evaporation method and then so patterned that the Au-Ge-Ni alloy remains in the contact holes of the insulating film 3. The remaining portions of the Au-Ge-Ni alloy serve as source and drain electrodes 7 and 8. As a result, the formation of the GaAs MESFET is completed.
As described above, the MESFETs are characterized by high frequency properties. Improvement in the high frequency of the MESFETs mean that the gate length L.sub.g will be shortened. Shortening the gate length L.sub.g to improve the high frequency property tends to minimize a section area of the gate electrode. The minimization of the section area of the gate electrode, however, tends to increase the gate resistance thereby rendering electric properties inferior. To recover such inferiority in the electrical properties due to the enlargement of the resistance, the low resistive metal film 6 is formed on the heat resistive metal film 12 having a high resistivity. The interface between the two metal films 12 and 6 making up the gate electrode is designed to have a large area, for example, a T-shaped section for reduction of the gate resistance.
Such T-shaped gate electrode is, however, associated with problems in the property of the high frequency and in the reliability. As described above, the heat resistive tungsten silicide film 12 is deposited by the sputtering or a magnetron sputtering. The use of the magnetron sputtering damages the channel layer. This results in inferiorities of the property of the high frequency and the reliability. Consequently, it is required to develop a novel method of forming a Schottky gate on the MESFETs free from the above problems.